This invention relates to an inorganic powder the individual particles of which are coated with a thin film of a polymer synthesized by plasma polymerization to improve the dispersibility of the powder which is scarcely dispersible in organic binders.
Industrial processes include operations which require dispersion of varied inorganic powders in organic binders. Inorganic powders, e.g., varied ceramic, ferrite, iron oxide, Co-doped iron oxide, and metal powders, are used for electronical functional devices, heat insulators, rubbers, magnets, magnetic tapes and discs, electrodes, and in many other applications. In an intermediate stage of the processes each inorganic powder must be dispersed in a polymeric material containing an organic solvent, i.e., in an organic binder. The quality of the end product is improved if the powder is dispersed thoroughly and uniformly. Inorganic powders are also added supplementarily, as reinforcing agent, filler, conductive additive or for other diversified purposes, to slurries containing organic binders as main ingredients. In these applications, too, the more uniformly the inorganic particles are dispersed the better the result.
Actually, however, thorough dispersion of inorganic powders in organic binders often involve great difficulties. This is because the inorganic substances are scarcely affinitive for the organic substances in which they are to be dispersed. Finer particles enhance the difficulty of uniform dispersion.
For the improvement of dispersibility it has been in practice to add a dispersent, such as a fatty acid ester, to the binder. However, the addition of a dispersant is not fully effective and, moreover, the dispersant can ooze out of the molded product to a disadvantage. Mechanical dispersion by a pearl mill, three-roll mill, ball mill, sand grinder mill or the like is also in use. Even this fails sometimes to ensure adequate dispersion. A serious shortcoming is that a prolonged treatment for mechanical dispersion on such a mill can break the minute shape of the inorganic particles. This possibility poses a major problem to the applications where the acicularity or other specific shape of the particles is an important consideration.
Under the circumstances, there has been wide-spread need for methods of satisfactorily improving the uniform dispersibility of inorganic powders in organic binders without any adverse effect upon the fine structure of the particles. In attempts to meet the requirements the following methods have hitherto been proposed:
(1) Coating inorganic particles with a dispersant.
(2) Coating inorganic particles with a high molecular compound, in the following ways:
(a) Coating with an aqueous high molecular compound. PA1 (b) Coating with an organic solvent type high molecular compound. PA1 (c) Coating by radiation polymerization. PA1 (d) Coating by ultraviolet polymerization. PA1 (e) Coating by any of mechanochemical techniques.
When inorganic particles are coated with a dispersant in accordance with the method (1), most of the dispersant simply sticks to the particle surface instead of being adsorbed on the latter in an effective way. Washing the dispersant off the surface would remove the smaller adsorbed portion too. If the particles are dispersed in the binder with the dispersant merely sticking to their surface, the proportional ratio of the dispersant to the binder will be so large that its unfavorable effects upon the quality of the end product will become evident. Furthermore, the operation will lack stability and continuity. For these reasons the method (1) is not desirable. For the coating with a high molecular compound (the method (2)), different approaches (a) to (e) have been proposed. However, none of them have, after all, succeeded in effecting uniform coating without uniformly dispersing the particles in the polymer solution. Ordinarily, such treatments are required because of the poor dispersibility of the particles in organic binders, and nevertheless the treatments themselves have to depend on good dispersibility of the particles for their success. This is, in a sense, a self-contradiction, and therefore the second method including their modifications are unacceptable. Another factor that hinders the commercial acceptance is that in those dispersion systems the inorganic powders and high molecular compounds tend to aggregate together for objectionable size enlargement, imparting undesirable effects upon the dispersion of the particles.
The polymeric coating films for the purpose of improving the powder dispersibility should be evenly and thinly formed on the particles. From this viewpoint none of the abovementioned methods of the prior art are satisfactory.
For the reasons stated, the development of a novel polymeric coating method to replace the foregoing has been called for. After our intensive studies about the method for polymeric coating of inorganic particles with a uniform and thin film in a stabilized operation, it has now been found that plasma polymerization best meets the end. The present invention is based upon this discovery.